It is known that blocked isocyanate crosslinkers are used as components in coating compositions, in conjunction with hydroxy-functional resins, to form urethane coatings on substrates. The urethane coating results once the coating composition is sufficiently cured. Illustrative urethane coatings include urethane powder coatings, urethane automotive base coatings, urethane automotive clear coatings, urethane electrocoatings, urethane primer coatings, urethane coil and wire coatings and the like.
It is also known that the blocked isocyanate crosslinkers require curing at elevated temperatures (e.g. greater than 320° F. and even greater than 350° F.) because, at the elevated temperatures, a blocking group associated with the crosslinker unblocks, i.e., removes itself, from the crosslinker and free isocyanate (NCO) functional groups remain. The free NCO functional groups are then capable of reaction with the hydroxy-functional groups of the resin to form a crosslinked network as the urethane coating.
Even with the elevated temperatures, the unblocking of the crosslinker is slow and, without a catalyst, typically results in urethane coatings that have a pool cure response such that the resultant coating is “underbaked” or “undercured”. As such, metal catalysts have been employed and function, with the elevated temperatures, to advance the unblocking of the crosslinker and to improve the cure response of the coating. Use of such catalysts also accounts for variations in curing temperatures which often result in the underbaked condition whereby a target temperature for cure of the urethane coating is not achieved.
The metal catalysts typically include metal oxides, such as tin oxide, dibutyl tin oxide, and bismuth oxide, and organo-metallic salts, such as bismuth carboxylate and dibutyl tin dilaurate. Whether a metal oxide or an organo-metallic salt, these metal catalysts are added, in an unmodified form, directly into the composition that forms the urethane coating. Examples of such conventional metal catalysts and such conventional additions of the metal catalysts are disclosed in U.S. Pat. Nos. 5,554,700; 5,670,441; 5,908,912; 5,972,189; 6,174,422; 6,190,524; 6,265,079; 6,333,367; 6,353,057; 6,436,201; 6,617,030; and 6,624,215
There are several deficiencies associated with this direct addition of the metal catalysts. It is difficult to directly add the metal oxides into the composition. Metal oxides frequently require intensive mechanical processes, such as grinding, to be effectively incorporated into the coating composition. As for the organo-metallic salts, in many instances, portions of the organo-metallic salts solubilize in the coating composition and, as a result, lead to certain physical defects, such as craters and/or poor film coalescence (realized as an undesirable ‘poor flow’ cracking-like phenomenon), in the cured coating. Frequently, portions of the organo-metallic salts are simply not compatible with the coating composition. Also, these types of metal catalysts, such as the specialized metal carboxylates disclosed in U.S. Pat. No. 6,353,057, are based on fatty acid ligands formed from low molecular weight carboxylic acids. While the ligands of the '057 patent are sufficient for complexing with the metal, such as the bismuth, it is known that they can have deleterious effects on the final, i.e., cured coating.
For example, if the particular carboxylic acid used in the '057 patent is of low molecular weight, e.g. an Mn less than about 200 Daltons, and is also at least partially soluble in water, then the carboxylic acid can cause contamination which is realized as craters in the cured coating. More specifically, in the art of electrocoating a substrate, it is typical for an e-coat ‘bath’, which contains the coating composition, to be filtered through an ultrafilter to provide an aqueous medium that is later used to rinse the substrate. When the bath is filtered through the ultrafilter, the ultrafiltrate, i.e., the portion of the bath that passes through the filter, is the aqueous medium. It is contemplated that low molecular weight carboxylic acids, such as those of the '057 patent, pass through the ultrafilter and contaminate the aqueous medium. This is undesirable because, in preparing a particular substrate, such as a body component of a vehicle, the substrate is sprayed with the aqueous medium to rinse the substrate. During spraying, the low molecular weight carboxylic acids which contaminate the aqueous medium, can also be sprayed onto the substrate thereby introducing a potentially crater-causing material on the substrate.
On the other hand, if the particular carboxylic acid used in the '057 patent is of high molecular weight, e.g. an Mn more than about 500 Daltons, then it can remain in the cured coating and cause problems during formation of the cured coating, i.e., during film formation, and also cause problems associated with adhesion of the cured coating to metal. The specialized, low molecular weight, metal carboxylates of the '057 also tend to exhibit poor stability stemming from addition of the metal carboxylates, such as a bismuth carboxylate, to an aqueous acidic medium. In this situation, the potential to hydrolyze exists and this potential is undesirable.
Thus, there remains a need to improve catalysis of reactions which form urethane coatings.